Regarding the sunset at Sounio, before deciding, do check some reviews, and candid guidebooks like Rough Guide. One of the problems is that this place is SO popular with bus tours, that around Sunset there can be 20+ busses with hundreds of people jamming the site. Many people have recommended going to Sounio instead in early AM for the morning light, then going to the beach just below, but that doesn't fit your schedule.
How about instead, walking along the pedestrianized promenade on South Side of Acropolis, (view on this map -- click & it gets huuuuge - http://www.orangesmile.com/destinations/img/athens-map-big.jpg ), and climbing up the hill called the Pnyx? (on this map it's called Pnika). This is where assemblies were held during Athens' golden age, where Pericles gave his famous orations; and it gives a wonderful view of the Acropolis turning gold in the sunset.
As for returning your van at Airport and then taking a group of newcomers with all their luggage and their important stuff (cash, credit cards, passports) on their person, and put them on the Athens Metro --- I say no no no. Athens is one of the safest cities in all Europe, but the two places that pickpockets target are the airport & seaport) Metro lines. Cars get crowded, someone bumps you and whoops! they're out the door and your wallet is missing. You cannot watch everyone and despite warnings, newbies WILL keep valuables in pockets or backpacks. Instead, take group to airport curb and catch the X95 Express bus to Syntagma Square. Goes every 20 minutes, costs Less, takes same time as metro (if you figure in the time required to reach Metro platform & then at end get from lower level up to street). You buy ticket at kiosk near bus stop - be SURE yr people "Validate" tickets by stamping them in a pole-mounted box inside bus. Bags are piled in central pen inside bus. No one can board bus after airport & only 2 stops = no pickpocketing. Another difference is that when Metro reaches Syntagma, there's a mad rush to push your way off before doors close; by contrast, bus terminates curbside at Syntagma, allows ample time to get bags and exit to sidewalk. I don't know how far Syntagma bus stop will be from your hotel, because you haven't given its name ... but distance would be same from Metro.